The present invention relates to carbon black for a tire tread rubber, and more particularly to carbon black which can impart a high abrasion resistance to a tire tread rubber while keeping the heat build-up in the tire tread rubber on a conventional level.
The effect of reinforcing a rubber by carbon black has heretofore been considered to be largely governed by the specific surface area (particle size) and structure of carbon black. Accordingly, there are known many grades of carbon black differing in these properties.
In compounding carbon black into a rubber component, an appropriate choice is made of the grade of carbon black having characteristics adapted to the application of a rubber composition to be prepared. For example, a hard grade of carbon black, such as N110 or N220, is used in a rubber member requiring a high abrasion resistance, such as a tire tread portion subject to severe running conditions. As the running conditions for tires have recently become more and more severe, however, such a high performance has been required of a tire tread portion so as to satisfy particularly a high abrasion resistance and a low heat build-up at the same time.
In general, it is known that the abrasion resistance of a tire tread portion is enhanced as the specific surface area and structure of carbon black compounded thereinto are increased. However, it also is known that the heat build-up in the tire tread portion is increased in keeping with increases in the specific surface area and structure of carbon black. Thus, the abrasion resistance has an antinomic relation with the heat build-up. Accordingly, it has been considered extremely difficult to simultaneously impart a high abrasion resistance and a low heat build-up to a rubber composition.
With a view to solving such a difficulty, there was proposed an attempt to impart a high abrasion resistance and a low heat build-up to a tire tread by using furnace carbon black having a nitrogen adsorption specific surface area (N.sub.2 SA) of 94 to 145 m.sup.2 /g, a dibutyl phthalate absorption of 89 to 107 ml/100 g, and a true specific gravity lower than the value calculated according to the equation: {(-0.0006.times.N.sub.2 SA (m.sup.2 /g)+1.8379} (see Japanese Patent Publication No. 34,149/1978). There also was proposed an attempt to simultaneously impart a high abrasion resistance and a low heat build-up to a tire tread by using carbon black satisfying the following three conditions: N.sub.2 SA=100.about.200 m.sup.2 /g T=.DELTA.Dst.sup.50 .times..DELTA.Dst.sup.75 .R, and 4,100-18.1 (N.sub.2 SA)&lt;T&lt;7,700-25.5 (N.sub.2 SA), wherein .DELTA.Dst is a difference between two equivalent Stokes diameters and R is the maximum absorbance at a Stokes mode diameter, to improve a workability in dispersing it into a rubber (see Japanese Patent Laid-Open No. 112,638/1988). Despite such proposals, however, no rubber compositions containing, compounded thereinto, carbon black having such specified properties can simultaneously secure the satisfactory levels of abrasion resistance and heat build-up, in which further improvements has therefore been demanded.
In view of the above, the inventors of the present invention have made investigations on not only the N.sub.2 SA and dibutyl phthalate absorption of carbon black but also the intraaggregate void volume thereof, and have found that, when carbon black in the form of aggregates having a certain level of structure and a large intraaggregate void volume is compounded into a rubber component, the resulting rubber composition has a high abrasion resistance and a low heat build-up. The present invention has been completed based on this finding.